JRuby's IRB sessions seem to not handle LocalJumpError exceptions in the main thread properly. In an MRI IRB session, the following happens:

while true
  return 1
end

#LocalJumpError: unexpected return
#       from (irb):2
#       from /usr/local/rvm/rubies/ruby-2.0.0-p0/bin/irb:16:in `<main>'
#2.0.0-p0 :004 >

In JRuby's IRB session, JRuby either exits (Centos 6) or blocks indefinitely (Windows 7 x64). However, this works in JRuby IRB:

q = Thread.new do
  while true
    return 1
  end
end
q.join

#LocalJumpError: unexpected return
# jruby-1.7.4 :004 >

So I'd imagine IRB isn't catching the LocalJumpError in the main thread as it should?